1. Field of the Invention
The present invention relates to electrochromic devices which exhibit coloration and bleaching thereof at ambient temperature by control of the polarity of an induced electric field. More particularly, this invention relates to a device and method for providing an electrochromic device with a gradient of color, i.e., color of different intensity in different regions thereof.
2. Discussion of the Related Art
Electrochromic devices are devices in which a physical/chemical change produced in response to an induced electric field results in a change in the reflective (or transmissive properties) of the device with respect to electromagnetic radiations, e.g., uv, IR, and visible radiation. Such devices, one embodiment being shown as item 10 in FIG. 1, generally comprise a film of electrochromic material and an ion-conductive insulating layer which functions as an electrolyte layer. The film and the electrolyte layer are in surface contact with each other for exchange of ions between the electrochromic film and the electrolyte layer. Two conductive electrode layers in FIG. 1, at least one of them being transparent, are disposed on the opposite outer surfaces of the film and the electrolyte layer to provide means for applying a voltage across the combined thickness of the electrochromic film and the electrolyte layer. The electrode layers are provided on substrates, which substrates may be of a material such as glass. Depending on the ion providing and ion storage capacity of ion conductive layer, a counter electrode located between ion conductive layer and electrode layer may be used. The electrodes are provided with external electrical leads connected to a voltage providing source. Application of a voltage of proper polarity across the electrodes causes coloration of the electrochromic layer. By reversing the polarity of the applied voltage, the colored electrochromic layer will be uncolored (bleached). Changing from the bleached state to the colorless state or from the colored state to the bleached is termed "switching". The electrochromic material may be persistent in either its colored state or its non-colored state. By "persistent" is meant the ability of the material to remain, after removal of the electric field, in the absorptive state to which it is changed, as distinguished from a substantially instantaneous reversion to the initial state. The length of time a material is persistent is called its "open circuit memory" or simply "memory". In some embodiments, the coloration can be erased by simply short circuiting the electrodes through an external circuit, there being enough internally stored charge to supply the reversed voltage required to raise the coloration in electrochromic layer.
In such devices, the electrochromic film usually comprises an inorganic metal oxide material, most commonly a transition metal oxide, in particular: tungsten oxide. When tungsten oxide is the electrochromic material, the electrolyte layer is adapted to provide a positively charged light cation, preferably, a proton or a lithium ion. The electrolyte layer is generally a liquid electrolyte solution which comprises polymers or copolymers containing acidic groups such as polystyrene sulfonic acid or a solid compound like lithium chloride. The electrolyte layer also may be a gel like polyvinyl butyral-methanol doped with LiCl.
It would be desirable in some situations to have an electrochromic display device which exhibits gradations of color intensity in different regions of the device during operation of the device, for example, to provide a monochromatic "picture" based on the same color but different intensities thereof. It would further be desirable to provide an electrochromic device having a gradient band of coloration, e.g., a more intense coloration in a lower region as compared to its upper region. Such a device could find use as walls of an office that could be switched from the colorless state to a graded colored state at will to provide privacy. It could eliminate the combined glass wall/venetian blind combination often used in offices today. Advantageously it would be easier to keep clean. It further will be appreciated that it might be desirable to have an electrochromic device which includes a gradient band in which the upper portion is more intensity colored than the lower region. Such a device might be useful as the windshield of an automobile, for the windows or buildings or for sunglasses.
One electrochromic device that includes a gradient of color intensities was disclosed by the instant inventor in U.S. Pat. No. 4,923,289. This device includes two substrates and therebetween: one electrode layer; an electrochromic material; an ion conductive material; and another electrode layer in that order. The electrochromic material has a continuously decreasing thickness gradient in at least one region when measured normal to the plane of an electrode layer.